The effect of incident radiation on a crystal can be characterised as follows: EQU P=X.sup.(1) .epsilon.+X.sup.(2) .epsilon..sup.2 -( 1)
where
P=induced dipole moment PA1 .epsilon.=applied electric field PA1 X.sup.(1) =first order coefficient PA1 X.sup.(2) =second order coefficient
The first order term in Equation 1 defines energy absorption by the crystal whereas the second order term has proved of particular interest since by providing a crystal which optimizes the second order effect, it is possible to achieve sum and difference frequency generation including second harmonic generation and optical rectification. Applications include frequency doublers in which input laser radiation is frequency doubled by the crystal to provide a source of intense coherent radiation outside of the frequency range where high performance semiconductor lasers can operate. Other uses include parametric amplification and optical detection.
The majority of inorganic crystals have small non-linear optical coefficients. The second-order coefficient can be greatly increased in epitaxially grown non-centrosymmetric (lacking inversion symmetry) multi layer structures, but the coefficients are sensitively dependent on the growth parameters and are not easily controllable after growth.
Recently, investigations have been carried out into the optical response of epitaxially grown semiconductor 2-dimensional quantum wells. It has been found that the heterojunction discontinuities lead to sub-bands with energy spacings in the range of 10.sup.-3 -1 eV (corresponding to wavelengths of about 1 mm-1 .mu.m) depending on the material parameters and quantum well dimensions. When the quantum wells are essentially symmetrical, a large, first order absorption of input radiation has been reported hitherto (L. C. West and S. J. Eglash, "First observation of an extremely large dipole infrared transition within the conduction band of a GaAs quantum well", Appl. Phys. Lett. 46, p. 1157, 1985). In such symmetrical quantum wells, the second order effects are forbidden. However, it has been reported that second-order effects can be obtained if the inversion symmetry is removed by the application of an electrical field or by the growth of compositionally asymmetric wells. [L. Tsang, D. Ahn and S. l. Chuang, "Electric field control of optical second-harmonic generation in a quantum well", Appl. Phys. Lett. 52, P. 697 (1988); M. M. Fejer, S. J. B. Yoo, R. L. Byer, Alex Harwitt and J. S. Harris, Jr., "Observation of extremely large quadratic susceptibility at 9.6-10.8 .mu.m in electric-field-biased AlGaAs quantum wells", Phys. Rev. Lett. 62(9), p. 1041 (1989); M. K. Gurnick and T. A. DeTemple, "Synthetic non-linear semiconductors", IEEE J. Quantum Electronics QE-19, p. 791, 1983; and E. Rosencher, P. Bois, J. Nagle and S. Delaitre, "Second harmonic generation by intersub-band transitions in compositionally asymmetrical MQW's", Electronics Lett. 25, P. 1063 (1989); C. Sirtori, F. Capasso, D. L. Sivco, S. N. G. Chu and A. Y. Cho, "Observation of large second-order susceptibility via intersubband transition at .lambda..apprxeq.10 .mu.m is asymmetric coupled AllnAs/GaInAs quantum wells", Appl. Phys. Lett. 59, 2302 (1991)]. By engineering well widths and barrier heights, the energy levels can be tuned to create two and three level systems whose energy separation is matched to the incoming radiation, and the structure can be optimized for optical rectification or second harmonic generation, for example.
Such prior art arrangements will now be described in more detail with reference to FIGS. 1 and 2.
FIG. 1 shows a section through an essentially two dimensional quantum well formed in an AlGaAs/GaAs/AlGaAs double heterostructure wafer. A device is shown schematically and comprises a GaAs substrate 1 with a GaAs layer 2 arranged between overlying and underlying AlGaAs layers 3, 4. As is well known in the art, an essentially two dimensional electron gas is formed in the GaAs layer 2 as a result of the heterojunctions 2, 3 and 2, 4.
The potential across the potential well in the vertical direction z is shown in detail in FIG. 2a. It will be seen that the well is symmetrical since the structure is centrosymmetric. As a result, incident optical radiation tends to be absorbed between particular quantized energy states E1, E2, E3 permissible within the well as shown in FIG. 2a, but there are no second order effects.
As shown in FIG. 2B, an asymmetry can be produced by applying an electric field in the vertical direction z so that there is a potential gradient within the well. Similarly, as shown in FIG. 2C, an asymmetric quantum well can be produced by introducing an additional layer between layer 2 and one of the layers 3, 4. Thus, as shown in FIG. 1, if the alloy composition (fraction of Al in Al.sub.x Ga.sub.1-x As) in the region 3a is made less than that in region 3b, a stepped characteristic for the potential in the well is produced as shown in FIG. 2C.
Both of the asymmetric wells (FIGS. 2B and 2C) exhibit second order effects in response to incident radiation and thus can be used as components for frequency doublers or the like. However, these known asymmetrical wells suffer a disadvantage in that they are not readily tunable. For the well shown in FIG. 2B the tunability is limited by ionization which occurs at high fields. The configuration shown in FIG. 2C is not tunable and the asymmetry is set by the alloy composition and layer thickness achieved during epitaxial growth.